The invention relates generally to LED lamps and LED lighting, and more particularly to LED lamps suitable to replace a fluorescent lamp in a luminaire having a ballast for use with fluorescent lamps.
Fluorescent lighting has been around for many years now. This form of lighting started out as a highly efficient alternative for incandescent light bulbs, but has recently been surpassed by LED lighting to some extent in terms of efficiency and power consumption, and also in other aspects as set out below.
An LED (light emitting diode) is a semiconductor light source, e.g. with a PN-junction which emits light when activated. Typically, when a suitable voltage is applied, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the colour of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs for lighting purposes are often combined with one or more phosphors to broaden or change the emitted spectrum of the LED device.
Fluorescent lamps generally comprise a tube filled with an inert gas and a small amount of mercury, capped at both ends with double pinned end caps (thus four pins in total). To facilitate the starting conditions and to limit current through the fluorescent lamp during operation, an electrical ballast is connected between the mains power supply and the fluorescent lamp. When fluorescent lighting was first introduced, simple inductors for limiting AC current were used as ballasts, usually referred to as magnetic ballasts. More recently, luminaires for fluorescent lamps are usually provided with electronic ballasts.
Electronic ballasts comprise electronic elements which typically first convert AC mains voltage into a DC voltage, and subsequently convert the DC voltage into a high frequency AC voltage to drive the fluorescent lamp. Electronic ballasts typically control and maintain current actively through the fluorescent lamp as constant current sources.
LED lighting has many advantages. For example, no mercury is required for LED lighting, LED lighting is more directional, LEDs require less effort to control or regulate power consumed, and the lifetime is greatly increased over fluorescent lighting. More importantly, LED lighting saves more power than fluorescent lighting.
It is thus desirable to replace fluorescent lamps by LED lamps. However, an LED lamp typically operates differently when used with different ballasts. In some cases, a straightforward replacement of a florescent lamp by an LED lamp results in a failure of the entire luminaire.
Currently, there are LED lamps on the market shaped like fluorescent tubes that can be placed in an existing fluorescent luminaire. However, these LED lamps require the luminaire to be stripped of the ballast and re-wired to directly connect the LED lamp to a mains power supply without intervention of a ballast, or replaced by a ballast designed for LED lamps. The labour required for stripping and re-wiring the luminaire negates much, if not all, of the savings involved in switching to LED lighting, or even presents higher costs.
Since the knowledge of ballasts is beyond the knowledge of an ordinary consumer, there is a need for an LED lamp that is compatible with any existing luminaire designed for a fluorescent lamp, so that the LED lamp can be put into operation when mounted in such a luminaire without needing to determine the type of the ballast in the luminaire.
The inventors found, surprisingly, that a compatibility issue even exists among electronic ballasts, in particular when the LED lamp requires less energy than the fluorescent tube it is designed to replace.
LED lamps typically consume about 30% less energy compared to fluorescent lamps which produce substantially the same amount of light. A recent trend is a move to more than 50% energy saving, e.g. an LED lamp that operates at lower than 29 W to replace a 58 W fluorescent tube.
However, the inventors observed that, once the LED tube is designed to operate at such a low power (e.g. designed to save 50% energy), some designs of electronic ballast behave anomalously. In some cases this results in the lifetime of the ballast being unacceptably reduced, e.g. to a few hundred minutes.
In this anomalous situation, when the LED lamp is installed in a luminaire having such a ballast, the current drawn from the ballast is considerably increased, which in turn results in an increase of temperature in elements in the ballast. Consequently, these ballast elements become too hot, such that the lifetime of the ballast is reduced (e.g. due to thermal runaway in critical components in the ballast) and possibly causing safety issues.
The inventors observed that ballasts having this anomalous behaviour are designed to operate as constant power sources, rather than constant current sources.
FIG. 1 shows a fluorescent lamp 101 arranged in a luminaire having a constant power ballast.
The luminaire is connected to an AC voltage source 102 (e.g. 230 V/50 Hz) and electronic components 103, which typically comprise a rectifier (which may be arranged in the ballast). The constant power ballast may comprise one or more inductors L1, L2, and one or more switches SW1, SW2 arranged to generate an output at a substantially constant power and at frequency in the range of e.g. 20 kHz-50 kHz.
A typical fluorescent lamp 101 has four electrodes (pins) for releasably connecting to the luminaire. Typically, the luminaire does not discriminate between different electrodes of the fluorescent lamp 101, so that the fluorescent lamp 101 does not have to be connected to the luminaire in a specific direction. After inserting the lamp in the luminaire, the two connections which supply the AC voltage from the ballast are called hot ends, and the other two connections (connected to capacitor 104) are called cold ends. In FIG. 1, the hot ends are labelled by letters A and D, and the cold ends are labelled by letters B and C.
A constant power ballast is designed to deliver a (nominally) constant amount of power to the lamp. The nominally constant amount of power may for example be derived from multiplication of the voltage drop across the lamp arrangement and the amount of current flowing through the lamp arrangement. When the fluorescent lamp is replaced by an LED lamp arrangement designed for operating at a lower power and the power consumed by the LED lamp arrangement is below the designed power of the ballast, these ballasts are designed to increase the current supplied to the LED lamp arrangement to reach the designed power. These ballasts are hereinafter called “constant power ballasts” for distinguishing from the majority of electronic ballasts, which are hereinafter called “constant current ballasts” as they are designed to maintain a substantially constant current output.
Constant power ballasts are typically designed to operate at a power that is significantly higher than the designed operation power of the energy saving LED lamp (e.g. designed to save 50% energy). Consequently, when fitting such an LED lamp in the luminaire, the constant power ballast determines that the power is too low (e.g. by determining that the voltage across the lamp is too low), so the ballast increases the current supplied to the LED lamp to reach the designed power output of the ballast. In the end, the current is too high and results in shortening the life or failure of the ballast.